We have investigated the C 1s photoelectron spectra of ethylene on the Si(1 0 0)(2 × 1) surface, using high resolution photoelectron spectroscopy. The vibrational structure has been observed in the C 1s spectra where the C-H stretching mode is dominant. The vibrational splittings of C 1s ionized state are very similar to those in the ground state. It is found that linear coupling model is applicable in this system and this excitation process is a Frank-Condon process. The curvature of potential energy surface of C-H and C-D bond is almost unchanged upon core ionization.